Control device for controlling drive force that operates on vehicle

ABSTRACT

A control device for controlling a front wheel drive force and a rear wheel drive force of a vehicle comprises a first controller for controlling a drive force of a main drive wheel and a drive force of an auxiliary drive wheel, the drive force of the main drive wheel being one of the front-wheel drive force and the rear-wheel drive force, and the drive force of the auxiliary drive wheel being another of the front-wheel drive force and the rear-wheel drive force; a second controller for sending to the first controller an auxiliary-drive-wheels-limiting drive force for limiting the drive force of the auxiliary drive wheels in a case that the vehicle is traveling in an unstable state; and a third controller for controlling a motor drive force, which is a source of the drive force of the main drive wheel and the drive force of the auxiliary drive wheel. The third controller reduces the motor drive force on the basis of the auxiliary-drive-wheel-limiting drive force.

FIELD OF THE INVENTION

The present invention relates to a control device (drive force controldevice) for controlling front wheel drive force and rear wheel driveforce of a vehicle.

BACKGROUND OF THE INVENTION

Vehicles, e.g., automobiles, generally have four wheels; i.e., two frontwheels and two rear wheels, and can have an electronic control devicefor driving the wheels.

Japanese Laid-Open Patent Application (JP-A) No. 2006-256605 discloses afour-wheel-drive electronic control unit (4WD-ECU) as such an electroniccontrol device. The 4WD-ECU disclosed in JP 2006-256605 A together witha vehicle stability assist (VSA)-ECU controls the drive force that actson the vehicle; and, specifically, sets the four-wheel-drive force interms of units of, e.g., torque.

A 4WD-ECU thus operates in coordination with the VSA-ECU and controlsthe drive force. Specifically, the VSA-ECU can request the 4WD-ECU tolimit the drive force in the case that, e.g., the vehicle is travelingin an unstable state. The 4WD-ECU can reduce the drive force and improvevehicle stability in response to a request from the VSA-ECU.

A VSA-ECU or other vehicle behavior control means can generally beprovided with at least one function from among a function forsuppressing spinning of the wheels (traction control system), a functionfor suppressing locking of the wheels (antilock brake system), and afunction for suppressing lateral sliding of a vehicle.

The vehicle is provided with motor control means (e.g., engine-ECU) asan electronic control device for controlling the motor drive force,which is a drive force source, and the engine-ECU, 4WD-ECU, and VSA-ECUcontrol the drive force overall.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a control device thatcan improve the stability of a vehicle.

Other objects of the present invention will be apparent to those skilledin the art in the description of a plurality of modes and preferredembodiments below with reference to that attached diagrams.

A number of aspects from among the plurality of aspects pursuant to thepresent invention will be described below in order to facilitateunderstanding of the general overview of the present invention.

According to the present invention, there is provided a control devicefor controlling a front-wheel drive force and a rear-wheel drive forceof a vehicle, the control device comprising: first control means forcontrolling a drive force of a main drive wheel and a drive force of anauxiliary drive wheel, the drive force of the main drive wheel being oneof the front-wheel drive force and the rear-wheel drive force, and thedrive force of the auxiliary drive wheel being another of thefront-wheel drive force and the rear-wheel drive force; second controlmeans for sending to the first control means anauxiliary-drive-wheel-limiting drive force for limiting the drive forceof the auxiliary drive wheel in the case that the vehicle is travelingin an unstable state, and third control means for controlling a motordrive force, which is a source of the drive force of the main drivewheel and the drive force of the auxiliary drive wheel, wherein thethird control means reduces the motor drive force on the basis of theauxiliary-drive-wheel-limiting drive force.

The second control means sends to the first control means anauxiliary-drive-wheel-limiting drive force for limiting the auxiliarydrive wheel drive force in the case that the vehicle is traveling in astate of, e.g., oversteer and is therefore unstable. The first controlmeans may reduce the auxiliary drive wheel drive force in the case thatthe first control means receives a request or instruction from thesecond control means. Oversteer or other instability can thereby besuppressed or eliminated. At this time, the main drive wheels can slipdue to an increase in the main drive wheel drive force accompanied by areduction in the auxiliary drive wheel drive force. Nevertheless, thethird control means reduces the motor drive force on the basis of theauxiliary-drive-wheel-limiting drive force, whereby the amount ofincrease in the main drive wheel drive force is reduced. Slipping of themain drive wheel can thereby be suppressed. Therefore, an improvement isrealized in regard to, e.g., the stability of the vehicle.

Preferably, the first control means increases the drive force of themain drive wheel by causing the drive force of the auxiliary drive wheelto match the auxiliary-drive-wheel-limiting drive force.

The auxiliary drive wheel drive force is reliably reduced to theauxiliary-drive-wheel-limiting drive force, whereby oversteer or otherinstability can be suitably suppressed or eliminated.

According to the invention, the second control unit may further have adetection unit for detecting whether the traveling state is unstable.

When the traveling state has been detected to be unstable by thedetection unit, the second control means sends anauxiliary-drive-wheel-limiting drive force to the first control means,and the third control means can reduce the motor drive force. In otherwords, when the second control means initiates a request to the firstcontrol means, the third control means can initiate a reduction of themotor drive force, which is based on the auxiliary-drive-wheel-limitingdrive force.

Preferably, the second control means has a first calculation unit forcalculating the auxiliary-drive-wheel-limiting drive force; and a secondcalculation unit for calculating a reduced drive force that representsan amount of reduction of the motor drive force on the basis of theauxiliary-drive-wheel-limiting drive force.

The second control means calculates the auxiliary-drive-wheel-limitingdrive force and can calculate a reduced drive force.

According to the invention, the reduced drive force may decrease incorrespondence with an increase in at least one of longitudinalacceleration and lateral acceleration of the vehicle.

The reduced drive force (the amount by which the motor drive force isreduced) can be set at a low level in the case that the longitudinalacceleration of the vehicle is high. The reduced drive force can be setat a low level in the additional case that the lateral acceleration ofthe vehicle is high. Change in the motor drive force is reduced andvehicle stability is improved by setting the reduced drive force to alow level.

According to the invention, the second calculation unit may calculate asthe reduced drive force a value obtained by multiplying a coefficientand a difference between the drive force of the auxiliary drive wheeland the auxiliary-drive-wheel-limiting drive force; and the coefficientmay decrease in correspondence with an increase in at least one oflongitudinal acceleration and lateral acceleration of the vehicle.

In the case that the longitudinal acceleration of the vehicle is high,the coefficient is set to be small, and the reduced drive force (theamount by which the motor drive force is reduced) can therefore be setat a low level. In the case that the lateral acceleration of the vehicleis high, the coefficient is set to be small, and the reduced drive forcecan therefore be set at a low level. Change in the motor drive force isreduced and vehicle stability is improved by setting the reduced driveforce to a low level.

Preferably, the second control means sends the reduced drive force tothe third control means; and the third control means may reduce themotor drive force by an amount equal to the reduced drive force.

The second control means sends auxiliary-drive-wheel-limiting driveforce to the first control means, and the second control means can sendreduced drive force to the third control means. The first control meansreduces the auxiliary drive wheel drive force, and the third controlmeans reduces the motor drive force. The control device can suppress oreliminate oversteer or other instability while suppressing slippage of amain drive wheel.

According to the invention, the drive force of the main drive wheel maybe the front-wheel drive force, and the drive force of the auxiliarydrive wheel may be the rear-wheel drive force.

In the case that the vehicle is traveling in a state of, e.g., oversteerand is therefore unstable, the rear-wheel drive force (auxiliary drivewheel drive force) is reduced, the front-wheel drive force (main drivewheel drive force) is increased, and oversteer can be reduced oreliminated.

According to the invention, the first control means is a drive forcecontrol means, the second control means is a vehicle behavior controlmeans, and the third control means is motor control means.

Persons skilled in the art can readily understand that each of aplurality of embodiments in accordance with the present invention can bemodified without departing from the spirit of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Certain preferred embodiments of the present invention will be describedin detail below, by way of example only, with reference to theaccompanying drawings, in which:

FIG. 1 is a diagrammatical view showing a general configuration of avehicle provided with a control device according to the presentinvention;

FIG. 2 is a block diagram showing a general configuration of the controldevice according to the present invention;

FIGS. 3(A) and 3(B) are graphical representations of examples of theoutputs from a calculation unit of the control device;

FIG. 4(A) and 4(B) are graphs each showing a control maps used forsetting a down coefficient; and

FIGS. 5(A) and 5(B) timing charts illustrative of operations of thecontrol device according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The embodiments described below are used for facilitating understandingof the present invention. Therefore, persons skilled in the art shouldnote that that the present invention is not unduly limited by theembodiments described below.

1. Vehicle

FIG. 1 diagrammatically shows a general configuration of a vehicleprovided with a control device according to the present invention. Avehicle 1 (e.g., an automobile) is provided with a control device 100capable of executing various controls, as shown in FIG. 1. The controldevice 100 is capable of controlling the front wheel drive force (thetarget value of the drive force transmitted to front wheels 71, 72) andthe rear wheel drive force (the target value of the drive forcetransmitted to rear wheels 73, 74) of the vehicle 1 as examples of thevarious controls. Specific control of the control device 100 accordingto the present invention is described below in “2. Control Device.”

In the example of FIG. 1, the vehicle 1 is provided with a motor 10(e.g., gasoline engine or another internal combustion engine), the motor10 has an output shaft 11, and the motor 10 can cause the output shaft11 to rotate. The vehicle 1 is provided with motor control means 20(e.g., an engine ECU) for controlling the motor 10, and a throttleactuator 21. The motor control means 20 obtains the motor drive force(target value), and the motor control means 20 controls the throttleactuator 21 so that the rotation (the actual motor drive force) of theoutput shaft of the motor 10 matches the motor drive force (targetvalue).

The throttle (not shown) position for controlling the amount of air-fuelmixture flowing into the motor 10 is controlled based on the motor driveforce via the throttle actuator 21. In other words, the motor controlmeans 20 obtains the throttle position that corresponds to the motordrive force, generates a control signal that corresponds to the throttleposition, and sends the control signal to the throttle actuator 21. Thethrottle actuator 21 adjusts the throttle position in accordance withthe control signal from the motor control means 20.

The vehicle 1 is provided with an accelerator pedal 22 and anaccelerator sensor 23. The accelerator sensor 23 detects the amount ofoperation of the accelerator pedal 22 by the driver of the vehicle 1 andsends the amount of operation of the accelerator pedal 22 to the motorcontrol means 20.

The motor control means 20 generally obtains the throttle position orthe motor drive force on the basis of the amount of operation of theaccelerator pedal 22. The vehicle 1 is provided with an engine speedsensor 24 and a pressure sensor 25. In the case that the motor 10 is,e.g., an engine, the engine speed sensor 24 can detect the engine speed,and the pressure sensor 25 can detect the absolute pressure inside theintake tube that takes the air-fuel mixture into the engine. The motorcontrol means 20 can obtain the throttle position or the motor driveforce on the basis of the amount of operation of the accelerator pedal22, and the detected absolute pressure and engine speed. The motorcontrol means 20 can modify the amount of operation of the acceleratorpedal 22 on the basis of a control signal (e.g., the traveling state ofthe vehicle 1) from the control device 100. Alternatively, the motorcontrol means 20 may obtain the motor drive force and the throttleposition on the basis of the amount of operation of the acceleratorpedal 22, the detected engine speed, the detected absolute pressure, anda control signal from the control device 100.

In the example of FIG. 1, the vehicle 1 may be provided with a powertransmission apparatus (power train, drive train). The powertransmission apparatus has, e.g., a transmission 30, a frontdifferential gear mechanism 51, front drive shafts 52, 53, a transfer54, a propeller shaft 55, a rear differential gear mechanism 61, reardrive shafts 64, 65, as shown in FIG. 1. The transmission 30 has atorque converter 31 and gear mechanism 32.

The power transmission apparatus is not limited to the example of FIG.1, and it is also possible to modify, revise, or implement the exampleof

FIG. 1. The power transmission apparatus may be, e.g., the drive forcetransmission system 3 disclosed in FIG. 2 of Japanese Laid-open PatentApplication (JP-A) No. 07-186758.

The rotation (the actual motor drive force) of the output shaft of themotor 10 is converted to actual all-wheel drive force (the actualfront-wheel drive force and the rear-wheel drive force) via the powertransmission apparatus. In the control related to such conversion, theall-wheel drive force is determined based on the motor drive force(target value) of the motor control means 20, the amplification factor(target value) of the torque converter 31, and the speed-change gearratio (target value) of the gear mechanism 32. The distribution from thefront-wheel drive force (target value), which is the main drive wheeldrive force, to the rear-wheel drive force (target value), which is theauxiliary drive wheel drive force, is determined based on thefront-wheel drive force (target value) and the distribution ratio of therear differential gear mechanism 61.

In the case that the distribution ratio of the rear differential gearmechanism 61 is, e.g., 100:0 in terms of the front-wheel drive force torear-wheel drive force, the front-wheel drive force (target value),which is the drive force of the main drive wheel, matches the all-wheeldrive force (target value). In the case that the distribution ratio ofthe rear differential gear mechanism 61 is, e.g., (100-x):x in terms ofthe front-wheel drive force to rear-wheel drive force, the front-wheeldrive force (target value), which is the main drive wheel drive force,matches the value obtained by subtracting the rear-wheel drive force(target value), which is the drive force of the auxiliary drive wheel,from the all-wheel drive force (target value).

The front wheels 71, 72 are controlled by the front-wheel drive force(target value) via the front differential gear mechanism 51 and thefront drive shafts 52, 53. The rear wheels 73, 74 are controlled by therear-wheel drive force (target value) via the rear differential gearmechanism 61 and the rear drive shafts 64, 65. The actual all-wheeldrive force is transmitted to the propeller shaft 55 via the transfer54, and a portion of the actual all-wheel drive force transmitted to thepropeller shaft 55 is distributed to the actual rear-wheel drive forcetransmitted to the rear differential gear mechanism 61. The remainingportion of the actual all-wheel drive force transmitted to the propellershaft 55, transfer 54, and front differential gear mechanism 51 is theactual front-wheel drive force.

In the example of FIG. 1, the vehicle 1 is provided with transmissioncontrol means 40 (e.g., an automatic transmission (AT) ECU) forcontrolling the speed-change ratio (e.g., the speed-change gear ratio ofthe gear mechanism 32) of the transmission 30. The vehicle 1 is providedwith a shift lever 33 and a shift position sensor 34, and thetransmission control means 40 generally determines the speed-change gearratio of the gear mechanism 32 on the basis of the shift position (e.g.,“1,” “2,” “D”) of the shift lever 33 as detected by the shift positionsensor 34.

In the case that the shift position of the shift lever 33 is, e.g., “1,”the transmission control means 40 controls the gear mechanism 32 so thatthe gear mechanism 32 has a speed-change gear ratio that represents thefirst speed. In the case that the shift position of the shift lever 33is, e.g., “D,” the transmission control means 40 determines thespeed-change gear ratio that represents any one among all of thespeed-change gears constituting the gear mechanism 32 composed of, e.g.,first speed to fifth speed, on the basis of a control signal (e.g., thespeed of the vehicle 1 and the all-wheel drive force (target value))from the control device 100. In accordance therewith, the transmissioncontrol means 40 controls the gear mechanism 32 so that the gearmechanism 32 has a speed-change gear ratio that represents any one of,e.g., the first to fifth speeds. For example, when the transmissioncontrol means 40 thereafter changes from, e.g., the speed-change gearratio that represents the first speed to the speed-change gear ratiothat represents the second speed, the transmission control means 40controls the gear mechanism 32 so that the gear mechanism 32 changesfrom the speed-change gear ratio that represents the first speed to thespeed-change gear ratio that represents the second speed.

In the example of FIG. 1, the vehicle 1 is provided with a wheel speedsensor 81 for detecting the rotational speed of the front wheel 71, andis also provided with a wheel speed sensor 82 for detecting therotational speed of the front wheel 72. The vehicle 1 is provided with awheel speed sensor 83 for detecting the rotational speed of the rearwheel 73, and is also provided with a wheel speed sensor 84 fordetecting the rotational speed of the rear wheel 74. The control device100 can obtain the speed of the vehicle 1 on the basis of the rotationalspeed (wheel speed) detected by the wheel speed sensors 81, 82, 83, 84.The vehicle 1 is provided with a longitudinal acceleration sensor 85(e.g., longitudinal G sensor for detecting acceleration in units ofgravitational acceleration) for detecting the acceleration of thevehicle 1 along the front-rear or longitudinal direction of the vehicle1, and the control device 100 can correct the speed of the vehicle 1using the acceleration.

In the example of FIG. 1, the vehicle 1 is provided with a yaw ratesensor 86 for detecting the yaw rate when the vehicle 1 turns. Thevehicle 1 is also provided with a lateral acceleration sensor 87(lateral G sensor for detecting the centrifugal acceleration in units ofgravitational acceleration) for detecting the centrifugal force(centrifugal acceleration) of the vehicle 1 along the lateral directionof the vehicle 1. The vehicle 1 is provided with a steering wheel 88 anda steering angle sensor 89, and the steering angle sensor 89 detects thesteering angle of the steering wheel 88.

The control device 100 can detect side slipping and other behavior ofthe vehicle 1 on the basis of the yaw rate, centrifugal acceleration(lateral acceleration), and steering angle. In addition to detectingsuch behavior, the control device 100 can carry out various controls(e.g., control related to at least one among the front wheels 71, 72 andthe rear wheels 73, 74 via the brakes or other braking unit (notshown)), and all of the controls described above are not required to becarried out. Described below is a general overview of control of thecontrol device 100.

2. Control Device

FIG. 2 shows in block diagram a general configuration of the controldevice according to the present invention. The control device 100 canreceive, e.g., the yaw rate, the steer angle, the wheel speed, thelongitudinal acceleration, and the lateral acceleration as inputsignals; can generate, e.g., two output signals; and can execute variouscontrols, as shown in FIG. 2. The control device 100 is provided withdrive force control means 300, and the drive force control means 300controls the drive force of the main drive wheels (e.g., the front wheeldrive force) and the drive force of the auxiliary drive wheels (e.g.,rear wheel drive force), as an example of the various controls.

In the example of FIG. 2, the control device 100 is provided withvehicle behavior control means 200. The vehicle behavior control means200 is capable of calculating as an example of various controls.Specifically, the vehicle behavior control means 200 calculates, e.g.,the auxiliary-drive-wheels-limiting drive force for limiting the driveforce of the auxiliary drive wheels. The vehicle behavior control means200 can send the auxiliary-drive-wheels-limiting drive force to thedrive force control means 300.

In the example of FIG. 1, the control device 100 is not provided withthe motor control means 20, but in the example of FIG. 2, the controldevice 100 may be provided with the motor control means 20. The controldevice 100 (not shown) may be furthermore provided with, e.g., thetransmission control means 40 of FIG. 1. In the example of FIG. 2, themotor control means 20 is capable of reducing the motor drive force onthe basis of the auxiliary-drive-wheel-limiting drive force calculatedby the vehicle behavior control means 200. In other words, the motorcontrol means 20 can receive a reduction request or instruction from thevehicle behavior control means 200.

Specifically, the drive force control means 300 determines the ratiobetween the main drive wheel drive force (target value) and theauxiliary drive wheel drive force (target value), for example, anddetermines, e.g., the auxiliary drive wheel drive force (target value)on the basis of the ratio and the all-wheel drive force (target value).The drive force control means 300 controls, e.g., the distribution ratioof the rear differential gear mechanism 61 of FIG. 1 using outputsignals so that the determined auxiliary drive wheel drive force (targetvalue) is obtained. The output signal from the drive force control means300 to the rear differential gear mechanism 61 is a control signal forcontrolling the auxiliary drive wheel drive force (target value).

When the auxiliary drive wheel drive force is zero due to thedistribution ratio of the rear differential gear mechanism 61, in otherwords, when the propeller shaft 55 and the rear drive shafts 64, 65 arecut off from each other, the main drive wheel drive force (target value)or the front-wheel drive force matches the all-wheel drive force (targetvalue) in the example of FIG. 1. Alternatively, when the auxiliary drivewheel drive force is not zero due to the distribution ratio of the reardifferential gear mechanism 61, in other words, when the propeller shaft55 and the rear drive shafts 64, 65 are connected, the main drive wheeldrive force (target value) matches the value obtained by subtracting theauxiliary drive wheel drive force (target value) from the all-wheeldrive force (target value) in the example of FIG. 1.

In the example of FIG. 2, the control device 100 is provided withvehicle behavior control means 200. The vehicle behavior control means200 is capable of accepting input signals that express yaw rate and thelike acquired from the yaw rate sensor 86 of FIG. 1, for example. Thevehicle behavior control means 200 is capable of calculating theauxiliary-drive-wheels-limiting drive force in accordance with detectionof the unstable state of the vehicle 1, which is based on, e.g., the yawrate or the like.

In the case that the vehicle behavior control means 200 makes a requestor instruction for auxiliary-drive-wheels-limiting drive force (limitingdrive force) to the drive force control means 300, the drive forcecontrol means 300 reduces the auxiliary drive wheel drive force (targetvalue) and the drive force control means 300 increases the main drivewheel drive force (target value). At this time, the drive force controlmeans 300 matches the auxiliary drive wheel drive force (target value)to the auxiliary-drive-wheels-limiting drive force (limiting driveforce) to reduce the auxiliary drive wheel drive force (target value).Specifically, the drive force control means 300 controls the reardifferential gear mechanism 61 so that the auxiliary drive wheel driveforce is reduced by the distribution ratio of the rear differential gearmechanism 61. When the propeller shaft 55 and the rear drive shafts 64,65 are more weakly connected, the actual auxiliary drive wheel driveforce is reduced, and as a result, the actual main drive wheel driveforce is increased. Reduced auxiliary drive wheel drive force makes itpossible to, e.g., reduce oversteer. Therefore, the stability of thevehicle 1 is improved, for example.

The drive force control means 300 is capable of determining in advancethe main drive wheel drive force (target value) and the auxiliary drivewheel drive force (target value), reducing the auxiliary drive wheeldrive force (target value) determined in advance in accordance with thea request from the vehicle behavior control means 200, and increasingthe main drive wheel drive force (target value) determined in advance.

The vehicle behavior control means 200 is capable of calculating thereduced drive force that expresses the amount by which the motor driveforce is reduced on the basis of the auxiliary-drive-wheel-limitingdrive force. The motor control means 20 reduces the motor drive force inthe case that the vehicle behavior control means 200 sends a reduceddrive force to the motor control means 20. At this time, the motorcontrol means 20 reduces the motor drive force by amount equal to thereduced drive force. In other words, the drive force control means 20can calculate a value obtained by subtracting the reduced drive forcefrom the primarily determined motor drive force as the secondarily(ultimately) determined motor drive force. The motor control means 20 iscapable of outputting an output signal that is based on the motor driveforce. The output signal from the motor control means 20 to the motor 10is a control signal that corresponds to, e.g., the throttle position inthe motor 10 of FIG. 1. The output signal from the motor control means20 to the motor 10, e.g., an internal combustion engine, may be acontrol signal corresponding to a fuel injection amount determined by afuel injection device (not shown) inside the internal combustion engine.The drive force control means 300 may also be referred to as firstcontrol means for determining the main drive wheel drive force (targetvalue) and the auxiliary drive wheel drive force (target value), and thevehicle behavior control means 200 may be referred to as second controlmeans. The drive force control means 300 (first control means) primarilydetermines the main drive wheel drive force (target value) and theauxiliary drive wheel drive force (target value). The drive forcecontrol means 300 (first control means) may determine whether to respondto the request for limiting the auxiliary drive wheel drive force(target value) from the vehicle behavior control means 200 (secondcontrol means), and may reject the request for limitation. In the casethat the vehicle behavior control means 200 sends to the drive forcecontrol means 300 the auxiliary-drive-wheels-limiting drive force(limiting drive force), the drive force control means 300 (first controlmeans) can secondarily (ultimately) determine the main drive wheel driveforce (target value) and the auxiliary drive wheel drive force (targetvalue).

The motor control means 20 may be referred to as third control means fordetermining the motor drive force. The motor control means 20 is, e.g.,an engine ECU, fuel injection (FI)-ECU, or the like.

3. Vehicle behavior control means (second control means)

FIG. 2 also shows a schematic structural diagram of the vehicle behaviorcontrol means 200 according to the present invention. The vehiclebehavior control means 200 (second control means) is capable ofrequesting or instructing the drive force control means 300 (firstcontrol means) to reduce the auxiliary drive wheel drive force (targetvalue). In the example of FIG. 2, the vehicle behavior control means 200is provided with a detection unit 210, a first calculation unit 220, anda second calculation unit 230. The first calculation unit 220 is capableof calculating the auxiliary-drive-wheel-limiting drive force forlimiting the auxiliary drive wheel drive force.

3.1. Detection Unit

The detection unit 210 detects, e.g., the unstable state of the vehicle1 and can instruct the first calculation unit 220 so that the firstcalculation unit 220 outputs the auxiliary-drive-wheels-limiting driveforce. In the case that an unstable state has been detected, thedetection unit 210 can send to the first calculation unit 220 a signal(e.g., a signal expressing a binary “1” or high level) expressinginstruction or permission to output the auxiliary-drive-wheel-limitingdrive force. For example, the actual yaw rate obtained from the yaw ratesensor 86 and the scale yaw rate calculated based on the speed of thevehicle 1 and the steering angle are used to determine whether or notthe vehicle 1 is traveling in a stable state. Specifically, an unstablestate can be defined as when the difference between the actual yaw rateand the scale yaw rate (yaw rate deviation) is greater than apredetermined value. Also, an unstable state may be determined bysubjecting the yaw rate deviation to filter processing. It is alsopossible to correct or adjust the scale yaw rate using the lateralacceleration acquired from the lateral acceleration sensor 87.

The detection unit 210 can accept input of the steer angle from, e.g.,the steering angle sensor 89. Also, the detection unit 210 is capable ofcalculating the average of four rotational speeds (wheel speeds)detected by, e.g., the wheel speed sensors 81, 82, 83, 84 and obtain theaverage wheel speed Vaw_av of the drive wheels as the speed of thevehicle 1. Alternatively, the detection unit 210 calculates the averageof two rotational speeds (wheel speeds) detected by, e.g., the wheelspeed sensors 83, 84, and obtain or estimate the speed Vvh_es of thevehicle 1 as the speed of the vehicle 1.

The speed Vvh_es (estimated speed) of the vehicle 1 may include theapplication of an increasing limit and a decreasing limit to each of thewheel speeds of the rear wheels 73, 74 (auxiliary drive wheels) in orderto eliminate the effect of noise caused by vibrations and the like ofthe vehicle 1, for example. In other words, the first calculation unit220 is capable of correcting or adjusting the two rotational speeds(wheel speeds) detected by the wheel speed sensors 83, 84, calculatingthe average of the two rotational speeds (wheel speeds) thus correctedor adjusted, and obtaining or estimating the speed Vvh_es of the vehicle1. The speed Vvh_es (estimated speed) of the vehicle 1 may be estimatedusing another method.

The detection unit 210 is capable of sending to the first calculationunit 220 a signal that expresses whether the traveling state of thevehicle 1 is unstable, and is furthermore capable of sending to thefirst calculation unit 220 a signal that expresses, e.g., the yaw ratedeviation of the vehicle 1. In the case that the vehicle 1 is travelingin an unstable state, the first calculation unit 220 is capableoutputting the auxiliary-drive-wheel-limiting drive force to the driveforce control means 300.

3.2. First Calculation Unit

The first calculation unit 220 of FIG. 2 calculates anauxiliary-drive-wheel-limiting drive force on the basis of detection ofan unstable state of the vehicle 1. In the case that the vehicle 1 istraveling in an unstable state, the first calculation unit 220 or thevehicle behavior control means 200 is capable sending to the drive forcecontrol means 300 an auxiliary-drive-wheel-limiting drive force forlimiting the auxiliary drive wheel drive force.

FIGS. 3(A) and 3(B) represent output examples of the calculation unit.The solid line in the example of FIG. 3(A) represents theauxiliary-drive-wheel-limiting drive force calculated by the firstcalculation unit 220, and the dotted line shows the auxiliary drivewheel drive force determined by the drive force control means 300. Thefirst calculation unit 220 or the vehicle behavior control means 200does not send a limit of the auxiliary drive wheel drive force to thedrive force control means 300 until time T1. In other words, the outputfrom the first calculation unit 220 is a value (single-dot-dash line)that does not limit the auxiliary drive wheel drive force. The valuethat does not limit the auxiliary drive wheel drive force is, e.g., amaximum value of the auxiliary drive wheel drive force that can bedetermined by the drive force control means 300. At time T1, the firstcalculation unit 220 requests from (outputs to) the drive force controlmeans 300 a limit (a value for limiting the auxiliary drive wheel driveforce; the auxiliary-drive-wheel-limiting drive force) of the auxiliarydrive wheel drive force. In the example of FIG. 3(A), the drive forcecontrol means 300 accepts a request from the first calculation unit 220at time T1 and causes the auxiliary drive wheel drive force to match theauxiliary-drive-wheel-limiting drive force. In other words, the driveforce control means 300 can cause the primarily determined auxiliarydrive wheel drive force to match the auxiliary-drive-wheel-limitingdrive force, and can use the auxiliary-drive-wheel-limiting drive forceas the secondarily (ultimately) determined auxiliary drive wheel driveforce.

The amount of reduction in output from the drive force control means 300at time T1 is a value obtained by subtracting theauxiliary-drive-wheel-limiting drive force from the primarily determinedauxiliary drive wheel drive force. The drive force control means 300receives a request from the first calculation unit 220 at time T1 andcauses the auxiliary drive wheel drive force to match theauxiliary-drive-wheel-limiting drive force, and the main wheel drivewheel drive force therefore increases by an amount commensurate with thedecrease in auxiliary drive wheel drive force. Oversteer or otherinstability can thereby be suppressed or eliminated.

The first calculation unit 220 is capable of calculating theauxiliary-drive-wheel-limiting drive force from time T1 to time T2 sothat, e.g., the yaw rate deviation is reduced so as to stabilize thetraveling state of the vehicle 1.

The traveling state of the vehicle 1 is not unstable at time T2, and thefirst calculation unit 220 or the vehicle behavior control means 200does not send a limit of the auxiliary drive wheel drive force to thedrive force control means 300 at time T2 and thereafter. In the exampleof FIG. 3(A), in a case in which the yaw rate deviation has decreasedat, e.g., time TA prior to time T2 and the state in which the vehicle 1is traveling is substantially stable, the drive force control means 300accepts requests from the first calculation unit 220 from time T1 totime TA and causes the auxiliary drive wheel drive force to match theauxiliary-drive-wheel-limiting drive force as indicated by the solidbold line in FIG. 3(A). In the example of FIG. 3(A), the drive forcecontrol means 300 accepts requests from the first calculation unit 220from time T1 to time TA and causes the auxiliary drive wheel drive forceto match the auxiliary-drive-wheel-limiting drive force as indicated bythe solid bold line in FIG. 3(A), in cases in which the yaw ratedeviation is smaller at, e.g., time TA prior to time T2 and thetraveling state of the vehicle 1 is essentially stabilized.

In the example of 3(B), at time T1, the first calculation unit 220calculates the auxiliary-drive-wheel-limiting drive force on the basisof the detection of the unstable state of the vehicle 1, and from timeT1 to time T2, the first calculation unit 220 continues to output theauxiliary-drive-wheel-limiting drive force, which is a fixed value thatdoes not depend on change in the yaw rate deviation after time T1. Inthe example of FIG. 3(B), the drive force control means 300 acceptsrequests from the first calculation unit 220 from time T1 to time T2 andcauses the auxiliary drive wheel drive force to match theauxiliary-drive-wheel-limiting drive force, which is drawn with a solidbold line.

In the examples of FIGS. 3(A) and 3(B), slipping of the main drive wheeldrives (e.g., the front wheels 71, 72) may occur due to an increase inthe main drive wheel drive force in the interval of time T1 to time TAor time T2. The vehicle behavior control means 200 of FIG. 2 may executethe function (traction control system) for suppressing spinning of thefront wheels 71, 72 and the rear wheels 73, 74. The vehicle behaviorcontrol means 200 or a second calculation unit 230 (described later) cancontrol spinning via a request or the like to reduce the motor driveforce on the basis of amount of slippage of the main drive wheels (frontwheels 71, 72). The vehicle behavior control means 200 or the secondcalculation unit 230 may suppress spinning via the brakes (not shown) orother braking unit. The slip amount Smw of the main drive wheels is avalue obtained by, e.g., subtracting the estimated speed Vvh es of thevehicle 1 from the average wheel speed Vmw av of the main drive wheels.In the case that the main drive wheels (front wheels 71, 72) slip in theinterval from time T1 to time T2, the vehicle behavior control means 200may request a limit of the auxiliary drive wheel drive force and mayalso request, e.g., a reduction in the motor drive force from the motorcontrol means 20.

3.3. Second Calculation Unit

The second calculation unit 230 of FIG. 2 is capable of calculating thereduced drive force that expresses the amount by which the motor driveforce is reduced on the basis of the auxiliary-drive-wheel-limitingdrive force. Specifically, the second calculation unit 230 can calculatethe reduced drive force of when the vehicle 1 has been detected to betraveling in an unstable state by the detection unit 210, the reduceddrive force being the difference between the auxiliary drive wheel driveforce (e.g., the auxiliary drive wheel drive force immediately prior totime T1 in FIG. 3(B)) from, e.g., the drive force control means 300 andthe auxiliary-drive-wheel-limiting drive force (e.g., theauxiliary-drive-wheel-limiting drive force immediately after time T1 inFIG. 3(B)) from the first calculation unit 220. In the example of FIG.3(B), the motor drive force may be reduced even after the main drivewheels (front wheels 71, 72) have slipped in the interval from time T1to time T2; however, the second calculation unit 230 may send a reduceddrive force to the motor control means 20 at time T1. Slipping of themain drive wheels (front wheels 71, 72) can thereby be suppressed at anearly stage at time T1.

The second calculation unit 230 is capable of calculating the reduceddrive force at time T1 by multiplying a coefficient and, e.g., a valueobtained by subtracting the auxiliary-drive-wheel-limiting drive forceat time T1 from the auxiliary drive wheel drive force at time T1. Thecoefficient is a coefficient (down coefficient) in the range of, e.g.,“0” to “1.” The down coefficient can be set lower in correspondence to ahigher level of at least one of the longitudinal acceleration or thelateral acceleration. The reduced drive force at time T1 is lower incorresponding fashion with the lower down coefficient. The downcoefficient may be suitably set in accordance with the attributes (e.g.,weight, engine displacement) of the vehicle 1.

FIGS. 4(A) and 4(B) represent examples of setting the down coefficient.In the example of FIG. 4(A), the down coefficient is higher incorrespondence with a higher longitudinal acceleration; however, therelationship between the down coefficient and the longitudinalacceleration is not limited to the example of FIG. 4(A). For example,the relationship between the down coefficient and the longitudinalacceleration may also be a line chart. The relationship between the downcoefficient and the longitudinal acceleration may be a curve expressedby a quadratic function, a higher-degree polynomial function, or thelike rather than a linear function; and may be a stepped linearrelationship expressed by a step function. In the example of FIG. 4(B),the down coefficient is higher in correspondence with a higher lateralacceleration; however, the relationship between the down coefficient andthe lateral acceleration is not limited to the example of FIG. 4(B). Forexample, the relationship between the down coefficient and the lateralacceleration may also be a line chart. The relationship between the downcoefficient and the lateral acceleration may be a curve expressed by aquadratic function, a higher-degree polynomial function, or the likerather than a linear function; and may be a stepped linear relationshipexpressed by a step function.

In the case that the second calculation unit 230 obtains the downcoefficient on the basis of both the longitudinal acceleration and thelateral acceleration at time T1, the down coefficient is the smallestamong the down coefficient (first down coefficient) based on, e.g., thelongitudinal acceleration and down coefficient (second down coefficient)based on the lateral acceleration. In the case that the longitudinalacceleration or the lateral acceleration of the vehicle is high, thedown coefficient, and consequently the reduced drive force, can be setto a low level with consideration given to the stability of the vehicle1.

The second calculation unit 230 can thus calculate the reduced driveforce at, e.g., time T1 when the traveling state of the vehicle 1 isunstable.

4. Motor Control Means (Third Control Means)

FIGS. 5(A) and 5(B) represent examples of the operation of the controldevice according to the present invention, and FIGS. 5(A) and 5(B)correspond to a reduction in auxiliary drive wheel drive force from timeT1 to time T2 of FIG. 3(B). In the example of FIG. 3(B), the firstcalculation unit 220 of FIG. 2 outputs theauxiliary-drive-wheel-limiting drive force to the drive force controlmeans 300 at time T1. At this time, the second calculation unit 230outputs the reduced drive force to the motor control means 20. In theexample of FIG. 5(A), the motor control means 20 accepts a request fromthe second calculation unit 230 at time T1 and causes the motor driveforce to be reduced by an amount equal to the reduced drive force. InFIG. 5(A), the solid line represents the change in the motor driveforce, and the motor drive force decreases from time T1 to time T2. Thismeans that the drive force of all the wheels (main wheel drive force andauxiliary drive wheel drive force) decreases from time T1 to time T2.The dotted line represents a comparative example, and in the case thatthe motor control means 20 does not receive a request from the firstcalculation unit 220, the motor drive force is constantly fixed, whichmeans that the drive force of all the wheels is also constantly fixedfrom time T1 to time T2.

In FIG. 5(B), the solid line represents the change in the main drivewheel drive force, and the main drive wheel drive force increases fromtime T1 to time T2. Since the main drive wheel drive force is a valueobtained by subtracting the auxiliary drive wheel drive force(auxiliary-drive-wheel-limiting drive force) from the drive force of allthe wheels, the amount of increase in the main drive wheel drive forcefrom time T1 to time T2 is a value obtained by subtracting the reducedamount of drive force of all the wheels from the reduced amount ofauxiliary drive wheel drive force. In FIG. 5(B), the dotted linerepresents a comparative example. In the comparative example, the amountof increase (dotted line) of the main drive wheel drive force from timeT1 to time T2 is greater than the amount of increase (solid line) of themain drive wheel drive force because the motor drive force is constantlyfixed and the amount of reduction of the drive force of all the wheelsis zero. This means that in the comparative example the main drivewheels may already be capable of slipping at time T1. In other words, inthe operation example of the control device 100, slipping of the maindrive wheels can be suppressed at an early stage at time T1 inaccordance with the reduction in the motor drive force carried out bythe motor control means 20.

Obviously, various minor changes and modifications of the presentinvention are possible in light of the above teaching. It is thereforeto be understood that within the scope of the appended claims theinvention may be practiced otherwise than as specifically described.

1. A control device for controlling a front-wheel drive force and a rear-wheel drive force of a vehicle, the control device comprising: first control means for controlling a drive force of a main drive wheel and a drive force of an auxiliary drive wheel, the drive force of the main drive wheel being one of the front-wheel drive force and the rear-wheel drive force, and the drive force of the auxiliary drive wheel being another of the front-wheel drive force and the rear-wheel drive force; second control means for sending to the first control means an auxiliary-drive-wheel-limiting drive force for limiting the drive force of the auxiliary drive wheel in the case that the vehicle is traveling in an unstable state, and third control means for controlling a motor drive force, which is a source of the drive force of the main drive wheel and the drive force of the auxiliary drive wheel, wherein the third control means reduces the motor drive force on the basis of the auxiliary-drive-wheel-limiting drive force.
 2. The control device according to claim 1, wherein the first control means increases the drive force of the main drive wheel by causing the drive force of the auxiliary drive wheel to match the auxiliary-drive-wheel-limiting drive force.
 3. The control device according to claim 1, wherein the second control unit further has a detection unit for detecting whether the traveling state is unstable.
 4. The control device according to claim 1, wherein the second control means has: a first calculation unit for calculating the auxiliary-drive-wheel-limiting drive force; and a second calculation unit for calculating a reduced drive force that represents an amount of reduction of the motor drive force on the basis of the auxiliary-drive-wheel-limiting drive force.
 5. The control device according to claim 4, wherein the reduced drive force decreases in correspondence with an increase in at least one of longitudinal acceleration and lateral acceleration of the vehicle.
 6. The control device according to claim 4, wherein the second calculation unit calculates, as the reduced drive force, a value obtained by multiplying a coefficient and a difference between the drive force of the auxiliary drive wheel and the auxiliary-drive-wheel-limiting drive force; and the coefficient decreases in correspondence with an increase in at least one of longitudinal acceleration and lateral acceleration of the vehicle.
 7. The control device according to claim 4, wherein the second control means sends the reduced drive force to the third control means; and the third control means reduces the motor drive force by an amount equal to the reduced drive force.
 8. The control device according to claim 1, wherein the drive force of the main drive wheel is the front-wheel drive force, and the drive force of the auxiliary drive wheel is the rear-wheel drive force.
 9. The control device according to claim 1, wherein the first control means is drive force control means, the second control means is vehicle behavior control means, and the third control means is motor control means. 